When light passes through the interface of two media having different refractive indices, part of the radiation is reflected. For instance, when light falls perpendicularly upon a glass pane, the reflected fraction of incident light is about 4% due to the difference between its refractive index of n=1.5 and the refractive index of air of n=1. The same fraction of about 4% is also reflected when the light exits from the glass. Thus, at the maximum, an amount of 92% of the incident light passes through a conventional glass pane, which can cause an undesired loss of efficiency, especially when a glass pane is employed to cover solar collectors or other optically sensitive elements. For this reason, precisely when it comes to the covering of solar collectors, it is desirable to use so-called antireflection-coated glass in which the radiant transmittance of the glass in question is enhanced by means of a coating on the surfaces.
In order to render glass anti-reflective, multiple layers can be applied onto the surface. In this process, on the basis of the interference principle, alternating layers having high and low refractive indices are applied. Owing to interferences of the partial waves that are reflected on the appertaining interfaces between the materials having different refractive indices, these partial waves are extinguished within a certain wavelength range, so that a particularly high radiant transmittance can be achieved for these wavelengths. Such alternating layer systems, however, are wavelength-selective and thus not suitable for use in a broadband spectrum. As a result, such coated glass is not suited for covering, for example, solar collectors, where it is crucial to achieve the best possible passage of light within the entire solar spectrum.
An alternative for rendering glass anti-reflective consists of applying a single layer onto the glass surface in question. Here, for physical reasons, an especially high transmission can be achieved if the surface layer has a refractive index equal to the square root of the refractive index of glass, in other words, a refractive index of about 1.22. In this case, the reflection of light having a wavelength that is four times the layer thickness is virtually zero, so that light having this wavelength is transmitted completely. Owing to the comparatively flat functional wavelength-dependence of the radiant transmittance, however, the latter is still particularly high for wavelengths that diverge from this. For this reason, precisely in the case of glass for covering solar collectors or other optically sensitive elements, efforts are aimed at obtaining a coating with a material that has a refractive index that is as close to 1.22 as possible.
Such a surface coating for glass can be produced by selectively etching the glass. For instance, etching soda-lime glass, for example, with hydrofluoric acid or hexafluorosilicic acid, can yield surface layers having a refractive index of around 1.27 which already comes very close to the desired result. The surface layers produced in this manner, in addition to good optical properties, also have relatively good mechanical properties, especially a high mechanical resistance to abrasion. Therefore, glass produced in this way is also fairly well-suited for everyday use. This production method has the drawback, however, that it calls for the use of acids that are extremely harmful to the environment and aggressive, which then requires correspondingly complex disposal measures and commensurate precautions for handling such materials.
As an alternative, the glass can also be coated by means of an additive application of coating material. On the one hand, high requirements have to be produced of coated glass made in this manner in terms of its optical properties, particularly with respect to a relatively small refractive index that is as close to 1.22 as possible. On the other hand, high requirements also have to be made of the mechanical properties of the coating of these types of glass, especially their abrasion resistance, in order to render them suitable for everyday use, even under relatively harsh conditions. With an eye towards these requirements, antireflection surface coatings on the basis of SiO2 particles have proven to be particularly well-suited.
In order to attain a suitably low refractive index of the surface layer that comes as close to n=1.22 as possible, the antireflection surface coatings on the basis of SiO2 particles are normally porous since an acceptably lower refractive index can already be achieved by merely thinning the coating material with air. Such porous antireflection surface coatings on the basis of SiO2 particles are normally characterized by more or less loose SiO2 particles joined together and having an essentially uniform particle size.
When glass is coated with such a porous antireflective surface coating on the basis of SiO2 particles, this is normally done using so-called sols in which [SiOX(OH)Y]n particles are mixed with solvents and optionally with a stabilizer. On the basis of such sols, coating solutions can be prepared into which the glass to be coated can be dipped, as a result of which the layer-forming sol precipitates onto the surface of the glass.
German patent DE 199 18 811 A1 discloses the use of such a sol on the basis of an alcohol-water mixture for the production of a porous antireflection surface coating on the basis of SiO2 particles. The antireflection surface coating produced here exhibits relatively good optical properties and is also sintering-stable so that the optical properties of an antireflection surface coating applied in this manner does not deteriorate to any appreciable extent, even during a subsequent thermal treatment of the coated glass, for example, in order to produce thermally toughened safety glass. However, for this coating, it has been found that the abrasion resistance does not meet the requirements for long-term use. For example, in the case of glass with such a porous antireflection surface coating, the test of the abrasion resistance according to DIN EN 1096-2 by means of the crockmeter test shows that marked damage to the layer already occurs after ten cycles and severe damage occurs after 100 cycles.
As an alternative, a porous antireflection surface coating on glass can also be produced using sols on the basis of aqueous systems that contain less than 1% organic components. The surface layers that can be produced by using such sols, which contain surfactants and which are essentially purely aqueous, increase the solar transmission of a low-iron soda-lime glass that is coated with such surface layers to as much as 95.3%, whereby the antireflection surface coating has a refractive index of 1.29. As it turned out, an antireflection surface coating produced in this manner is mechanically very stable and abrasion resistant, whereby the test of the abrasion resistance ascertained by means of the crockmeter test according to DIN EN 1096-2 revealed only slight changes to the layers even after 100 cycles. However, a drawback of antireflection surface coatings produced in this manner is that, due to the manufacturing process, the layers can exhibit inhomogeneities. Particularly in terms of visual appearance, crosswise streaking occurs that can be ascribed to periodical differences in the layer thickness within the range of a few nanometers. Such streaking can be detrimental. Moreover, the antireflection surface coatings that can be produced by using such an aqueous sol only yield unsatisfactory optical results when used for coating prism-cut glass, whereby the achievable radiant transmittance is only about 93.6%.